Recently, in the creation of shaped articles of noble metal with profound elements of industrial art, the practice of producing the shaped articles of noble metal aimed at by using a clay composition having the noble metal in a powdered form and an organic binder as basic components, shaping the clay composition in a predetermined form, drying the shaped clay composition and sintering the dry shaped article, thereby removing the binder composition as by dint of decomposition, vaporization or combustion, and inducing cohesion of the adjacent particles of the powdered noble metal has been in vogue.
As the conventional product mentioned above, the clay composition for shaping noble metal has been known to comprise a powdered noble metal having an average particle diameter in the range of 5 to 30 μm and containing as a main portion such particles of diameters as fall in the range of 1 to 100 μm and an organic binder formed of 0.02 to 3.0 wt % of starch and 0.02 to 3.0 wt % of a water-soluble cellulose resin.
A study that has substantiated low-temperature sintering by using powdered noble metals having different particle diameters has been proposed as disclosed in JP-A 2002-241802, for example.
The conventional clay composition for forming noble metal as described above, however, is such that while it has acquired fully satisfactory strength and restrained shrinkage successfully to a duly low level when it is sintered in a temperature range from the melting point of the noble metal to a temperature 250° C. lower than the melting point, it has been unable to acquire fully satisfactory strength when it is sintered at a temperature lower than the temperature range mentioned above. When an electric furnace that is capable of retaining the clay composition at a duly high temperature is used, it is made possible to acquire a sinter having fully satisfactory strength. The electric furnace of such a capacity as this, however, is very expensive. In contrast, an electric furnace for household use is small and simple and is mostly rather deficient in the ability to heat and in the control of temperature. Thus, it has been incapable of retaining the interior of the furnace at a high temperature or controlling the temperature accurately and, therefore, has failed at times to permit production of a sinter possessing fully satisfactory strength. For the sake of enabling the clay composition for shaping noble metal to produce a sinter having fully satisfactory strength, it has been necessary to widen the range of the sintering temperature adopted for it.
It has been heretofore known that this range of temperature can be widened by using a plurality of powders having different average particle diameters as found in the clay composition disclosed in JP-A 2002-241802 mentioned above. At least the clay composition of this publication, however, inevitably results in aggravating the shrinkage (shrinkage of about 12 to 20%) due to sintering. During the shaping of a form, therefore, it has been necessary to increase the size of this form by estimating the size obtained subsequent to the sintering, namely by giving due allowance for the shrinkage expected to take place. Especially when a product combining a ceramic and various decorative parts of metals is to be manufactured, an unduly large estimate of the shrinkage has possibly resulted in causing the decorative parts to loosen and fall off the clay part prior to the sintering. Conversely an unduly small estimate of this shrinkage results in preventing the shaping from producing a form aimed at and consequently suffering it to yield a warped form instead and eventually disrupting the pleasure of the shaping because the part of the clay adjoining the decorative parts deforms as by protuberating with a large shrinkage.
This invention is aimed at eliminating such problems as enumerated above and providing a clay composition for shaping noble metal that sinters effectively at temperatures in a wide range and induces only small shrinkage due to the sintering.